A dielectrically-loaded loop antenna is disclosed in British Patent Application No. 2309592A. Whilst this antenna has advantageous properties in terms of isolation from the structure on which it is mounted, its radiation pattern, and specific absorption ratio (SAR) performance when used on, for instance, a mobile telephone close to the user's head, it suffers from the generic problem of small antennas in that it has insufficient bandwidth for many applications. Improved bandwidth can be achieved by splitting the radiating elements of the antenna into portions having different electrical lengths. For example, as disclosed in British Patent Application No. 2321785A, the individual helical radiating elements can each be replaced by a pair of mutually adjacent, substantially parallel, radiating elements connected at different positions to a linking conductor linking opposed radiating elements. In another variation, disclosed in British Patent Application No. 2351850A, the single helical elements are replaced by laterally opposed groups of elements, each group having a pair of coextensive mutually adjacent radiating elements in the form of parallel tracks having different widths to yield differing electrical lengths. These variations on the theme of a dielectrically-loaded twisted loop antenna gain advantages in terms of bandwidth by virtue of their different coupled modes of resonance which occur at different frequencies within a required band of operation.
Dielectrically-loaded quadrifilar helical antennas are disclosed in British Patents Nos. 2292638, 2310543 and 2367429. Antennas in accordance with these patents have been used mainly for receiving circularly polarised signals from satellites of the Global Positioning System (GPS) satellite constellation for position fixing and navigation purposes. GPS is a narrowband service. There are other satellite-based services requiring receiving or transmitting apparatus of greater fractional bandwidth than that available from the prior antennas.
It is an object of the invention to provide improvements in bandwidth.